Microemulsions based on perfluoropolyethers have been long known and have been used in several applications, in particular for stabilizing dispersions of additives in lubricating oils or as additives in polymerization processes of fluorinated monomers.
Thus, U.S. Pat. No. 4,990,283 (AUSIMONT S.P.A.) 5 Feb. 1991, U.S. Pat. No. 5,656,201 (AUSIMONT S.P.A) 12 Aug. 1997, U.S. Pat. No. 5,670,088 (AUSIMONT S.P.A) 23 Sep. 1997, U.S. Pat. No. 5,698,138 (AUSIMONT S.P.A) 16 Dec. 1997 and U.S. Pat. No. 5,725,802 (AUSIMONT S.P.A) 10 Mar. 1998 disclose microemulsions comprising an aqueous medium, a perfluoropolyether typically having an average molecular weight ranging from 400 and 3000 and a fluorinated surfactant, the surfactant being typically selected among C5-C11 perfluorocarboxylic acids and salts thereof, and use thereof as additives in polymerization processes of fluorinated monomers.
Recently, perfluoroalkanoic acids having eight or more than eight carbon atoms have raised environmental concerns. Accordingly, efforts are now devoted to phase out from such compounds and methods have been developed to manufacture fluoropolymer products by aqueous polymerization procedures using alternative surfactants having a more favourable toxicological profile.
Fluorinated surfactants having general formula Rf—(OCF2CF2)k-1—O—CF2—COONH4, wherein Rf is typically a C1-C3 perfluoroalkyl group and k is typically 2 or 3, in particular CF3CF2OCF2CF2OCF2COONH4, which are known to be endowed with a bioaccumulation profile typically lower than that of perfluoroalkanoic acids having eight or more than eight carbon atoms, are notably used as polymerization emulsifiers in the aqueous emulsion polymerization of fluorinated monomers, as disclosed in JP 2006321797 A (ASAHI GLASS CO. LTD.) 30 Nov. 2006, JP 2007283224 A (ASAHI GLASS CO. LTD.) 1 Nov. 2007, WO 2008/132959 (ASAHI GLASS CO. LTD.) 6 Nov. 2008 and EP 2058291 A (ASAHI GLASS CO. LTD.) 13 May 2009. Nevertheless, emulsion polymerization processes disclosed therein fail to provide for small-sized polymer dispersed particles and high polymerization rates.
It would thus be desirable to find alternative routes for manufacturing fluoropolymers in a convenient and cost-effective way through fast reaction rates using equipments commonly used in the aqueous polymerization of fluorinated monomers with traditional fluorinated surfactants.